The invention relates generally to a high field magnet and, particularly, to a high field superconducting magnet having a wide bore for use in a nuclear magnetic resonance (NMR) spectrometer.
The technique of NMR has proven to be a powerful and unique tool for the study of complex molecular structures. High current density superconducting magnets are particularly well suited to provide the magnetic field uniformity and persistence required for NMR. As a result, a relationship exists between the available range of application of NMR spectroscopy, in field and sample volume, and the state of the technology of high current density superconducting magnets. Traditionally, increased field strength in high resolution NMR magnets has been sought for the study of the structure of molecules of increasing size. The number of spectral lines associated with larger molecules requires the increased line separation and sensitivity afforded by higher fields. Recently, unexpected benefits of high fields have been realized due to mechanisms of line width minimization at fields being approached in available spectrometer magnets. As a result, the motivation for increased field strength in NMR magnets is greater than ever. There are currently a number of programs under way with the objective of NMR at 1 GHz, corresponding to 23.5 T, and above. The possibility of these high fields depends, as a necessary condition, on the availability of a superconductor and associated coil technology for that field.
Given the scientific and commercial importance of NMR and the associated spectrometer magnets, there is motivation to address the technology of high current density superconducting magnets. More specifically, very high field NMR magnet technology is desired for instrumentation to support high field NMR research and to provide a wide bore 900 MHz spectrometer magnet. Such a magnet is also desired because the technology development activities directed toward the requirements of the 900 MHz magnet specifically are also applicable generally to high field NMR magnets, to high field, high current density superconducting magnets, and more generally to many aspects of magnet technology regardless of the type of conductor and construction being employed.
Moreover, a high field magnet with a larger bore than presently available is desired. Those skilled in the art believe that such a wide bore or large bore magnet will serve as an essential stepping stone to 1 GHz or higher frequency systems. Due to the high stored energy of the 900 MHz system and the associated large magnetic forces, however, the production of a successful system is challenging.
In general, a magnet of this type employs Nb3Sn and NbTi conductors in a set of epoxy impregnated long solenoids plus compensation coils for uniformity. The high field and large bore result in large mechanical stress in the coils and large magnetic stored energy. Therefore, reinforcement of the windings and an active protection system is desired.
Moreover, magnetic field uniformity is critical to NMR. Ferromagnetic welds cause field inhomogeneity. Historically, magnet designs have avoided ferrous structural alloys to prevent potential field distortions from welds. This strategy is problematic in fabricating high field magnets because austenitic stainless steel is the preferred heat treatment material for bore tubes in Nb3Sn coils. Early high field NMR designs employed the removal of the bore tubes after heat treatment and epoxy impregnation. Bore tube removal is dangerous due to the risk of damaging the reacted Nb3Sn conductor and leads. A more practicable fabrication approach is to leave the stainless steel bore tubes in place. For this reason, a weld metal on the coil form that avoids magnetic fields is desired.
A major obstacle to producing a wide bore, high field magnet involves the relatively large mechanical stresses caused by the magnetic fields in the magnet. Energizing a wound coil with an electric current produces a magnetic field accompanied by an associated mechanical stress in the coil. As the strength of the magnetic field produced by the coil increases, the magnitude of the mechanical stress increases as well. In this instance, a magnetic coil wound with superconductor produces a very high field and, thus, mechanical stresses become an important design factor.
In general, superconductors are composite materials in the form of flat tapes or wires (round or rectangular). The composite conductor typically includes copper or silver for protection and stabilization in addition to a superconducting alloy or compound. The composite conductor may also have substantial fractions of other materials (e.g., bronze). Unfortunately, the materials normally found in high field superconductors are generally of low strength and the high temperature heat treatment and annealing to which such conductors are subject diminishes their strength even further. For this reason, a magnetic coil structure providing sufficient strength to withstand the high mechanical stresses that appear in the windings of a high field magnetic coil is desired. There have been some attempts at high strength versions of superconductors, but even these materials would benefit from additional high strength supporting materials in high field magnet applications.
Magnetic coils used for the production of high magnetic fields are often cylindrical in form. In a cylindrical coil, there are two main components to the mechanical forces in the windings. First, a force in a radially outward direction generally tends to expand the diameter of the coil. Second, an axial force at each end of the coil toward the center results in a pressure at the midplane of the coil and tends to make the coil shorter. Both of these forces can produce excess mechanical stress on the conductor. Therefore, magnet reinforcement is desired for containing the radial component of the force to limit the radial expansion of the windings as well as containing the axial component of the force to reduce the pressure on the conductor at the center of the coil about the midplane.
Those skilled in the art are familiar with reinforcing a cylindrical magnetic coil by applying structural material to the outside surface of the coil. The added material forms a secondary cylindrical structure in contact with the cylindrical structure of the coil windings. For example, high strength wire wound into place over the magnetic coil provides reinforcement for the conductor in the coil. This construction has strength in the radial direction, against the expansion of the hoops formed by the reinforcement winding, but can be weak in the axial direction, where any spaces between the turns in the reinforcement winding reduce the stiffness in the axial direction. External reinforcement of this type may be applied without additional bonding material, relying on winding tension alone to hold the reinforcement winding in position, but is commonly applied along with a bonding material such as epoxy. The epoxy serves to fill any gaps between the turns in the reinforcement winding and to increase the stiffness of the reinforcement in the axial direction.
Those skilled in the art recognize that the forces or stresses on the magnet increase as the strength of the magnetic field increases. The so-called A15 high field superconductors, including Nb3Sn, are used to produce coils with the highest fields and forces but also tend to be the most brittle and subject to damage from mechanical stress. Unfortunately, the fabrication process for this type of coil places restrictions on the manner in which the reinforcement can be included in the design. One method of fabricating a high field superconducting coil, commonly referred to as “wind and react,” begins with winding the coil with an intermediate stage of conductor. The coil is then heat treated in a furnace at high temperature allowing the components of the intermediate stage conductor to react to form the final superconducting compound. The coil may then be finished by impregnation with an epoxy to secure the relatively weak, brittle superconducting wires and fragile insulation. Nb3Sn and the other A15 superconductors, for example, are referred to as “wind and react” conductors because they undergo a heat treating process to form the actual superconducting material.
The conventional process of externally reinforcing the coil involves applying the winding on the outside of the finished, epoxy impregnated coil. There are two major drawbacks to this method. In order to apply the reinforcement winding to the finished coil, after the coil is epoxy impregnated and essentially complete as an electrical winding, the coil must be refitted in the winding machine for the application of the reinforcement. This requirement is not severe for a small coil, but as the size of the coil increases for higher field magnets, this processing step becomes increasingly burdensome. Furthermore, this situation makes it difficult to achieve a strong bond between the reinforcement and the coil winding because the reinforcement winding is being applied over a completed, epoxy impregnated winding. The bond of fresh epoxy over already cured epoxy at the interface between the coil winding and the reinforcement winding will have a strength inferior to the shear strength within the windings themselves.
Although applying the reinforcement winding over the conductor winding after heat treatment, but before epoxy impregnation, would solve the problem of the epoxy bond strength, the conductor in the coil after heat treatment is sufficiently brittle that the application of the reinforcement before impregnation of the winding would present a large risk to the integrity of the conductor. Therefore, this option is not available.
For these reasons, improved externally reinforced windings to a high field superconducting coil that achieves the objective of providing structural reinforcement in the radial and axial directions and which is compatible with the other process requirements of these coils is desired.
Adequate quench protection presents another obstacle to producing high field magnets. Since superconducting magnets are designed to produce high magnetic fields, they store relatively large amounts of magnetic energy in normal operation. Superconducting magnets are subject to a mode of failure, known as “quench,” in which the stored energy is suddenly converted into heat accompanied by the presence of large electrical voltages. A quench occurs when there is a transition from the superconducting state to the normal state of the conductor in some region of the coil. In the normal state, the conductor has an electrical resistance and is heated by the current in the magnet. If the region is of limited size, and all the energy of the magnet is deposited in the region, the energy density is high and the region will be likely to overheat. The excessive heat and voltage during a quench can damage a magnet's windings. Although systems are known for protecting the magnet from damage due to a quench fault condition, these conventional systems are not well-suited for high field superconducting magnets such as those desired for NMR.
With some superconducting magnets, it is possible to remove the stored energy from the coil using an external dump resistor and switch. When a quench detector senses the quench condition in the magnet, a protective circuit opens the switch to essentially create a series circuit of inductor and resistor. The magnet largely deposits its stored energy in the external resistor as it decays with a time constant characteristic of such circuits. Although this type of protection system may be suitable for superconducting magnets that operate at relatively high current in powered mode, an external dump of energy is not practical for NMR spectrometer magnets that operate at relatively low current in persistent mode.
One alternative to removing the magnetic stored energy during a quench condition is to dissipate the energy internally to the magnet windings. A quench is usually a local phenomenon and, thus, the energy will dissipate locally. In this instance, the local region will overheat and be damaged if enough energy is available in the magnet. Distributing the energy somewhat uniformly over the entire volume of the magnet will help prevent overheating any one portion of the windings. Conventional protection systems are available for distributing the stored energy in the magnet. The particular type of system used depends on the type of magnet involved.
In a single coil magnet, which is a single, thermally-connected structure, conventional protection techniques involve electrically subdividing the coil into sections and providing a shunt path for each section. The shunt may consist of a resistor in parallel with the coil section, a diode in parallel with the coil section, or a series combination of a resistor and a diode in parallel with the coil section. In the event of a quench in one section, the current can shift into the shunt parallel with that section, and reduce the heating in the section that quenched. This hopefully will provide sufficient time for the quench to propagate by thermal conduction to the other sections of the coil, increasing the volume of the coil over which the heat is dissipated and thereby reducing the temperature. Spreading the quench of a superconducting coil throughout the coil in the event that one region of the coil quenches is the basic purpose and function of quench protection systems for magnets that are internally protected. Protection systems differ in the way these objectives are achieved. Unfortunately, use of the shunt path to spread the quench only works for coil sections that are thermally connected. In a magnet of multiple independent coils, the quench cannot thermally propagate to other coil sections.
Therefore, a circuit is desired for the protection of large magnets with large stored energy and risk associated with quench that is able to spread the quench of a superconducting coil throughout the coil in the event that one region of the coil quenches.
Yet another problem associated with conventional magnet design involves the leads of the superconducting coil. Mechanical stress on the lead wire extending from a coil, resulting from Local Lorentz forces or relative motion between the coil and the surrounding support structure, for example, can damage the lead wire. Moreover, certain known high field superconductors formed by a high temperature heat treatment are relatively brittle. Thus, the amount of bending allowed by the superconductor is very limited after heat treatment. For this reason, the conductor is often wound while it is still relatively ductile prior to heat treatment. The conductor, however, must be placed in a final position before heat treatment, held in that position during heat treatment, and kept free from bending after heat treatment. Therefore, it is necessary to position the lead from a superconducting coil during winding and to maintain that position during and after heat treatment until the lead can be formed into a structure designed to prevent it from being damaged.
U.S. Pat. No. 5,739,689, the entire disclosure of which is incorporated herein by reference, discloses a superconducting NMR magnet configuration. U.S. Pat. Nos. 5,690,991 and 4,744,506, the entire disclosure of which are incorporated herein by reference, teach superconducting joints for use in superconducting magnets.